The field of the invention is esters and processes of making the same from polycarboxylic acids. The present application is particularly concerned with a process for recovering the methanolic mother liquor (ML) obtained in the preparation of dimethyl terephthalate (DMT) when possibly unprepurified terephthalic acid (TPA) is esterified with methanol (M).
The state of the art of preparing dimethyl terephthalate may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol 15 (1968), pages 466-467, under the section "Current Commerical Processes for Polymer-Grade Dimethyl Terephthalate", and by reference to U.S. Pat. Nos. 2,876,252; 3,364,251; 3,399,227; 3,546,283; 3,607,044; and 3,617,226; and British Pat. Nos. 938,318; 1,043,289; and 1,053,164, the disclosures of which are incorporated herein.
Dimethyl terephthalate (DMT) is the raw material for the preparation of polyesters and especially for fibers, and because of this it is required to be of extremely high purity. On the other hand, commerical TPA generally also contains minute amounts of intermediate and side products. For instance, if TPA is prepared in known manner from p-xylol by air oxidation in the presence of heavy metal salts and bromine ions into acetic acid solution, then it contains slight amounts of impurities in the form of acetic acid (HAc), p-toluylene acid (TA), terephthal aldehyde acid (TPAA), benzoic acid (BA), p-hydroxymethylbenzoic acid, heavy metal salts, etc. Purification of the practically unmeltable and insoluble TPA is difficult. This applies most of all to separating the TPAA's. It was proposed in the prior art to apply air oxidation again for the crude TPA's in HAc at high temperatures above 200.degree. C. However, the oxidizing decay of the HAc's and corrosion are interfering in this respect and also the leaching out of the crude TPA's with HAc, possibly followed by catalytic hydrogenation and/or carrier vapor sublimation are quite unsatisfactory because of the large cost in equipment and because recovery of the mother liquor obtained by esterifying the TPA's to DMT is still required.
Therefore immediate esterification of the crude TPA's with methanol was proposed. When this is done, the aldehyde components must be removed from the DMT, which is feasible only by means of costly purification, for instance by mixing the crude DMT with a compound containing a reactive methylene group, for instance ethylacetoacetate, diethyl malonate, ethylcyanoacetate, phenylbenzylketone, etc., by heating in the presence of an alkaline catalyst and separation by distillation of the purified DMT from the aldehyde condensation products as disclosed in British Pat. No. 938,318. Also, the crude ester contaminated mostly with terephthalaldehyde acid methyl ester has been treated with molecular oxygen in the melt in the presence of a catalyst soluble in the reaction system, this ester subsequently being separated from the oxidized contaminations by distillation as disclosed in British Pat. No. 1,043,289. The expenditures involved in puifying crude DMT when carrying out this process on an industrial scale are described in Europ. Chem. News of Sept. 29, 1972, 32: where the crude ester is distilled, then recrystallized twice and distilled again (see also Chemie Ing. Technik, 35 193 ff (1963), No. 3).
The above listed major difficulties in the preparation of pure DMT may be easily circumvented if the esterification of powdery crude TPA is carried out with methanol in the vapor state in the presence of bulk esterification catalysts such as silica gel beads, as disclosed in U.S. Pat. No. 3,607,044 and U.S. Pat. No. 3,907,709, for instance in conformity with the process of U.S. Pat. No. 3,617,226 and U.S. patent application Ser. No. 39,761, and U.S. patent application Ser. No. and U.S. Pat. Nos. 3,940,431 and 3,886,200. In this process, the product flow of methanol, DMT, MMT, (monomethyl terephthalate), H.sub.2 O, TPAA methylester, TA methylester, BA methylester, etc. issuing from the esterification reactor is cooled stepwise, whereby one obtains a suspension of finely crystallized DMT in boiling CH.sub.3 OH. That is, the more easily soluble intermediary products such as TPAA methylester, MMT, TA methylester, BA methylester etc. are dissolved from the crystallizate and enriched in the mother liquor under the conditions of a steady recrystallization or new dissolution.
Esterification under thos conditions takes place within a few seconds, and the thermal loading of the reaction products thereby is slight. Therefore, the crude ester is of outstanding quality because of the additional steady recrystallization. The terminal purification of this crude ester even in the presence of an initial acid with high concentrations of imtermediary and side products requires merely another simple distillation. However, serious difficulties are encountered in recovering a mother liquor containing DMT as well as intermediate and side products both as regards the recovery of the pure methanol as the recovery of the valuable end and intermediate products. This applies especially to a mother liquor of the above esterification containing from about 1.0 to 6.0 percent solids in solution besides water and minor amounts of methylacetate following separation of the crystallized out DMT at a temperature of 10.degree. C, these dissolved solids being mostly DMT, MMT, TPAA merthylester, TA methylester and BA methylester. If this methanolic solution is subjected to recovery by distillation, appreciable complications are encountered: For instance the aldehyde compounds suffer from condensation reactions accompanied by the formation of products dyed markedly yellow and of no application (yellow oils). The necessary saponification of methylacetate yields acetic acid, so that high grade steel must be used. The occurrence of crystal sludge in the distillation sump and the crystallization out of DMT in the distillation column interfere with the recovery of the crystal sludge, which consists of various products, and causes apreciable cost expenditures.
These prior art problems are also described in U.S. Pat. No. 3,399,227, which recommends discarding the distillation resdiues.
U.S. Pat. No. 3,546,283 first teaches a commercially applicable process for recovering the mother liquor by saponifying the ester contained therein, by distilling off the methanol, by acid separation of the organic acids and their renewed use in the oxidizing stage. It is advantageous in this respect that ordinary steel equipment may be used, that the recovery of the substances dissolved in the mother liquor (oxidation end and intermediary products) appreciably increases the yield of the overall process, and that the interfering side-products are eliminated from the process. But there is the drawback of continued use of alkali and corresponding amounts of mineral acid.